This application claims the priority benefit of Taiwan application serial no. 89110673, filed Jun. 1, 2000.
1. Field of Invention
The present invention relates to an organic electro-luminescent device (OEL) and, especially to a flexible organic electro-luminescent device and the process thereof.
2. Description of Related Art
The organic electro-luminescent material has characteristics such as self-emitting, broad range of visual angle (0-160xc2x0), high response speed, low driving voltage, and full colors. It has been put into practice as a color plane display panel, such as a compact display panel, an out-door display billboard, a computer, and a television monitor. The organic electro-luminescent material has been developed since 1960. The organic electro-luminescent material usually is used to form a light emitting layer (EML). The light emitting layer incorporates between a metal electrode and a transparent anode, such that an organic electro-luminescent display is formed.
The organic electro-luminescent devices are divided into two groups according to the type of material used: one is a small molecule organic electro-luminescent device and a polymer organic electro-luminescent device. In the early 1980s, U.S.A. Eastman Kodak utilized tri-(8-hydroxyquinoline) aluminium (Alq3) to form an organic emitting layer and inserts a hole injecting layer between the emitting layer and the anode. This manner greatly improves the characteristics and stability of the organic electro-luminescent device, and launches the application of the organic electro-luminescent device. In 1990, Cambridge University of England utilized poly p-phenylene vinylene (PPV) conjugated polymer to fabricate a polymer organic electro-luminescent device. Since the materials of ploy(p-phenylene vinylene (PPV) type have the characteristics similar to semiconductors and the easy fabrication process for the polymer organic electro-luminescent devices, it highly interests people to make intensive researches again.
Since plastic has properties of transparency, light weight, flexibility, proper stretching strength and brittle resistance, plastic can be used as a substrate for a liquid crystal display (LCD) that is portable, thin and light. For example, the plastic substrate disclosed in U.S. Pat. Nos. 5,237,439 and 5,245,457 by Sharp Co. Ltd., Japan. The plastic substrate can also be used as substrates for the organic electro-luminescent devices (for example, U.S. Pat. No. 5,844,363 assigned to Princeton University, USA). The plastic substrates can be also applied in other optical display devices.
The material used for the plastic substrate is usually acrylic resin, epoxy resin, polyethylene terephthalate (PET), or polycarbonate (PC). However, the above materials used for the plastic substrate can not endure high temperature. Therefore, in such processes for producing liquid crystal displays and organic electro-luminescent devices, the temperature can not exceed 200xc2x0 C. when a transparent conductive electrode of indium tin oxide (ITO) is formed on the plastic substrate. A surface treatment of hard coating is also necessary to be performed to prevent the plastic substrate from being scraped, since the plastic material usually is soft.
Further, the plastic substrate can not effectively prevent water and oxygen from entering because of its low packing density, which also causes the absorption of water and oxygen. However, the organic film formed in the organic electro-luminescent device is very sensitive to the water and oxygen, so that the organic film would be damaged by the water and oxygen, which results in a decrease in the lifetime of the organic electro-luminescent device. Moreover, the water and oxygen contained in the plastic material are often released during vacuum deposition, causing that the evaporated layer has poor adhesion. Also, the water and oxygen are gradually released after the device is accomplished, resulting in deterioration of performance for the conductive electrode and luminescent material of the organic electro-luminescent device. The foregoing factors may cause poor performance and low stability of the plastic organic electro-luminescent device.
Referring to FIG. 1, a schematic view of the structure of the plastic organic electro-luminescent device in the art is shown. Such structure is disclosed in U.S. Pat. No. 4,885,211. The structure is fabricated by coating a transparent conductive electrode 102 on a substrate 100, where the transparent conductive electrode 102 serves as a hole injection layer. The conductive electrode 102 is formed of indium tin oxide, with the thickness of 30 nm to 400 nm and an area resistance of smaller than 100 xcexa9/cm2. Further, an organic emitting layer 104 is coated on the transparent electrode 102. Then, a metal conductive electrode 106 having a low work function, serving as an electron injection layer (EIL) is coated on the surface of the organic emitting layer 104. The material used for the metal conductive electrode 106 comprises Li, Mg, Ca, Al, Ag, In, or alloys thereof. The metal conductive electrode 106 has a thickness of 100 nm to 400 nm.
The organic electro-luminescent devices are generally divided into two types of a small molecule organic electro-luminescent device and a polymer organic electro-luminescent device according to the organic material used in the organic electro-luminescent device. The methods for coating the emitting layer of the organic electro-luminescent device can also be different.
The small molecule organic electro-luminescent layer usually has a two-layer structure, as described in U.S. Pat. No. 5,844,363 proposed by Princeton University, USA. A hole transport layer (HTL) having the thickness such as 80 nm and an emitting layer having the thickness of such as 80 nm are formed in sequence on the indium tin oxide layer by vacuum deposition. The material used for the hole transport layer comprises N,Nxe2x80x2-dipheny-N,Nxe2x80x2-(m-tolyl)benzidine (TPD). The material used for the emitting layer comprises tri-(8-hydroxyquinoline)aluminum (Alq3).
The polymer organic electro-luminescent layer usually has a single-layer structure, as described in the Synthetic Metals, 55-57, 4123-4127 (1993) published by G. Gustafsson et al. In such structure, a poly(2-methoxy-5-(2xe2x80x2-ethyl-hexyloxy)p-phenylene-vinylene (MEH-PPV) having a thickness of 50 nm to 100 nm is used as the emitting layer. In the polymer organic electro-luminescent device, the transparent conductive electrode is an indium tin oxide layer or a polyaniline (PANI) layer with camphor sulfonic acid (CSA) formed by spin coating, dipping, spray coating, doctor knife, screen printing, or inkjet printing.
However, either in the small molecule organic electro-luminescent device or the polymer organic electro-luminescent device, the organic electro-luminescent layer (for example, 104 in FIG. 1) and metal electrode (for example, 106 in FIG. 1) in these devices are very sensitive to water and oxygen. This results in a reaction with water and oxygen, such that these devices are damaged in the atmosphere that contains even a little amount of water and oxygen. Therefore, in the process for producing the organic electro-luminescent device, the demand for controlling the content of the water and oxygen in the atmosphere is strict, i.e. the required content of the water and oxygen therein is no more than 1 ppm. Further, U.S. Pat. No. 5,844,363 discloses a flexible small molecule organic electro-luminescent device formed by vacuum deposition, but it still failed to provide a solution to effectively prevent the water and oxygen from being released from the plastic substrate. Also, in the research published by G. Gustafsson et al., the flexible polymer organic electro-luminescent device is formed by spin coating, without any treatment for the plastic substrate.
For the plastic thin film liquid crystal display, the water and oxygen released or penetrated from the plastic substrate are necessarily to be avoided, so as to protect the plastic substrate. Also, the thermal stress between the plastic substrate and the indium tin oxide electrode is necessary to be reduced to prevent the layers from being cracked. Therefore, a protecting film layer must be coated between the indium tin oxide layer and the plastic substrate.
In U.S. Pat. No. 5,237,439, a hard coating layer having the thickness of 2 xcexcm to 6 xcexcm is formed by dipping and baking on both surfaces of the plastic substrate which has a thickness of 0.1 mm to 0.5 mm. The material used for forming the hard coating layer comprises organosilane, acrylic acid, melamine, and urethane, all of which are doped with boron. Such a hard coating layer can protect the plastic substrate and absorb the water released from the plastic substrate. The hard coating layer can also buffer thermal stress existing between the undercoat of SiOx with 10 nm to 60 nm in thickness, and the indium tin oxide electrode, so that cracks in the indium tin oxide layer can be avoided. Moreover, a TiOx buffer layer can also be formed between the organic hard coating layer without boron doping and the undercoat of SiOx to achieve the same properties with that of the boron-doped organic hard layer. The above TiOx buffer layer, SiOx undercoat and ITO electrode all are formed by sputtering deposition.
In U.S. Pat. No. 5,245,457, a method of forming topcoat in low temperature is provided. In such method, the commercially available silica-containing oil for coating is applied over the surface of the indium tin oxide electrode on the plastic substrate. After exposure to the radiation of UV light, a low temperature treatment is carried out in the temperature lower than 200xc2x0 C. to form a topcoat and prevent the plastic substrate from being damaged.
In U.S. Pat. No. 5,808,715, a titanium dioxide-silicon dioxide composite layer is provided to serve as the topcoat and undercoat of the liquid crystal display device, in which the titanium dioxide-silicon dioxide composite layer is formed as the topcoat and undercoat for the transparent electrode of the plastic thin film liquid crystal display by ion-assisted electron gun evaporation under a temperature lower than 100xc2x0 C. Such a titanium dioxide-silicon dioxide composite layer has superior insulating property, considerable hardness, and smooth surfaces. It has the following advantages: (1) preventing shortage between electrodes caused by conductive impurities having the same size as or larger than the gap between electrodes; (2) protecting plastic substrate from being scraped; (3) serving as a water and oxygen barrier layer; (4) serving as a bonding layer between the plastic substrate and indium tin oxide layer to prevent cracks of the layers caused by the difference in thermal expansion between indium tin oxide layer and the plastic substrate or by bending.
The present invention provides an organic electro-luminescent device and the process to fabricate the same. According to the present invention, a topcoat and undercoat are formed on the transparent conductive electrodes of the small molecule and/or polymer organic electro-luminescent device, respectively. The topcoat and the undercoat serve as the water and oxygen barrier layer and hard protecting layer for the transparent electrodes of the organic electo-luminescent device. The topcoat and the undercoat also serve as the protecting layer for the metal conductive electrode (i.e. electron injection layer) in the organic electro-luminescent device. The topcoat, undercoat and transparent conductive layer are designed to have the appropriate thickness ranges, so as to increase the luminescent efficiency of the organic electro-luminescent device.
According to the above and other objects of the present invention, a process for fabricating is provided, in which a transparent plastic substrate having a first surface and a second surface is provided. A first composite layer and a second composite layer are formed on the first and second surfaces of the transparent plastic substrate by low-temperature ion-assisted electron gun evaporation. A transparent electrode is formed on the first composite layer by low-temperature ion-assisted electron gun evaporation. An organic emitting layer of small molecule luminescent material is formed on the transparent electrode by thermal evaporation, or an organic emitting layer of polymer luminescent material is formed on the transparent electrode by spin coating. A metal electrode is formed on the organic emitting layer by thermal evaporation. Also and, a protecting layer is formed on the metal electrode by low-temperature ion-assisted electron gun evaporation to enclose the metal electrode and the organic emitting layer completely.
Further, according to the above and other objects, an organic electro-luminescent device is provided. The organic electro-luminescent device comprises a plastic substrate having a first surface and a second surface. A first composite layer and a second composite layer are formed on the first surface and the second surface of the plastic substrate, respectively. An indium tin oxide electrode is formed on the first composite layer. The first composite layer is located between the plastic substrate and the indium tin oxide electrode. An organic emitting layer of small molecule or polymer luminescent material is formed on the indium tin oxide electrode. A metal electrode is formed on the organic emitting layer so as to allow the organic emitting layer to be between the indium tin oxide electrode and the metal electrode. Additionally, a silicon dioxide protecting layer can be also applied on the metal electrode to enclose the metal electrode and the organic emitting layer completely.